This invention relates to electroacoustic converters and ferried transducers, such as used in ultrasonic devices, such as ultrasonic welders for the welding of thermoplastic or other materials. More specifically, this invention concerns improvements in the construction of a clamped transducer or converter utilizing a plurality of piezoelectric wavers clamped between two masses.
A typical one-half wavelength electroacoustic converter is disclosed in the co-assigned U.S. Pat. No. 5,590,866, and is shown in FIG. 1. This prior art electroacoustic converter C has a stack of piezoelectric ceramic wafers W clamped between front and back metal driver masses FM and BM. The piezoelectric materials may be a lead zirconate titante material and are oftentimes referred to as ceramic material. The piezoelectric wafers used in such transducers have a hole at their centers and their diametrical faces may be coated with an electrically conductive material (e.g., silver) to provide suitable electrical contact between the wafers. The piezoelectric wafers are energized with alternating electrical voltage from a suitable power supply and, when energized, the piezoelectric wafers mechanically vibrate. More specifically, as an alternating voltage is applied to the radially polarized piezoelectric wafers, the diameter of each disk or wafer alternatingly increases and decreases in response to the applied voltage. As a result of such diametrical changes, the thickness of the wafer also alternatingly mechanically increases and decreases which manifests itself as longitudinal vibrations. As the wafers vibrate, they in turn apply longitudinal mechanical vibrations to the front driver mass which may be coupled to a suitable horn or other ultrasonic tool for performing the desired work (e.g., the welding of thermoplastic workpieces). In a typical industrial apparatus employing such transducers, the predetermined frequency is typically, but not necessarily, in the ultrasonic range of, for example, 20 kHz., however, such frequencies can vary widely (e.g., between 1-100 kHz) depending on the application. Typically, the peak-to-peak longitudinal excursion of such vibrations is quite small, in the range of approximately 0.001 inches at 20 kHz, but can be increased by coupling the transducer to a suitably shaped horn.
Such devices convert high frequency electrical energy supplied by a suitable power supply into mechanical vibrations. The transducers have an output end to which an intermediate coupler (also oftentimes referred to as a booster horn) is generally attached, for receiving the vibrations from the converter and coupling the vibrations at the same or increased amplitude to an output horn, tool, sonotrode or the like which in turn couples the vibrations to a workpiece. Such half wavelength transducers often have a mounting flange located in a nodal area of the transducer where the vibrations are primarily in the radial direction.
Generally, most of the prior art converters based on the design shown in FIG. 1 have worked well for their intended purposes and have been designed to operate at various predetermined frequencies and power levels. However, certain limitations or shortcomings of such prior art converters have been widely known.
In general, the piezoelectric ceramic wafers and the metal components of the prior art converters have certain geometric shapes and these components have certain vibrational wave velocity properties. The planer stresses generated within these components while undergoing mechanical vibrations (as when the converter is resonant) and the velocity of these vibrations within the various components is not linear. This leads to certain problems or limitations with the prior art converter designs.
Typically, in order to achieve more power from the converter, the converters are designed for increased electrical capacitance. For any given voltage applied to the active elements of the converter (i.e., the piezoelectric wafers) to produce power, an electrical current is required. The current must be conducted through the capacitive branch of the circuit and results in a voltage rise across the piezoelectric wafers. In order to keep this voltage within permissible values and in order to increase the power of the transducer, more piezoelectric ceramic volume is needed. Typically, and as shown in FIG. 1, several pairs of piezoelectric ceramic wafers W are placed in parallel with one another so as to increase the total capacitance of the converter.
However, the increased volumes of ceramic material needed to achieve such higher power levels leads to problems. Typically, such designs use three pairs of ceramics wafers (as seen in FIG. 6) in order to produce the required higher power levels. This results in a larger volume of ceramic material relative to the volume (mass) of the converter as a whole. This larger volume or mass for the converter may result in the vibrational node for the converter not being located at desired location (typically within the mounting flange F, as shown in FIG. 1), but may be located within the ceramic wafers W. This results in unwanted motion and leads to lost energy.
In such prior art transducer designs, the ceramic wafers are driven in parallel. Since these prior art designs are so-called distributed designs, the equivalent motional voltages required to drive each pair of ceramic wafers may differ. This leads to unwanted circulating currents, electrical losses, and unequal power contributions for each pair of ceramic wafers.
Still further, as the frequency and stress within the ceramic wafers change, the location of the node point within the transducer will change and thus how the mechanical load is reflected back into the ceramics will change also. This changing condition makes the converter more unpredictable in its electrical impedance characteristics.
As is typical with any electromechanical transducer, the conversion of electrical power into mechanical energy (vibrations) generates heat within the transducer. The thermal resistance or thermal conductance of the ceramic components of the transducer is higher than that of the metal components. An increase in the volume of the ceramic components leads to areas within the transducer of high thermal concentrations and most of the heat transfer within the transducer takes place by conduction from the ceramic components to the metal components (primarily the front and back driver masses). Increased temperature of the ceramic components reduces their efficiency in converting electrical energy into mechanical energy.
Still further, if the volume of the ceramics is increased to result in higher desired power levels or to compensate for losses, the physical dimensions of the converter change, yet the converter dimensions are limited by the desired operating frequency. Thus, there must be a balance between the amount of ceramic material which can be provided to compensate for losses and the mechanical dimension of the converter.
It should also be recognized that there are mechanical losses in the ceramic material and in the mechanical structure for the converter. These losses occur at the operating fundamental frequency of the converter and sometime at harmonic frequencies of the fundamental frequency. In many prior designs, it is not unusual to excite third harmonic motions within the converter. These third harmonics typically occur in the mounting flange or in the back driver of the converter. Such harmonic motions produce no useful work and contribute to localized losses and temperature increases.
Among the several objects and features of the present invention may be noted the provision of an electroacoustic converter in which the electrical and mechanical characteristics of the converter are more stable and are less influenced by variations in operating conditions of the converter;
The provision of such an electroacoustic converter which produces higher power than prior converters of similar size;
The provision of such an electroacoustic converter in which the internal losses are reduced;
The provision of such an electroacoustic converter which is better controlled than prior converters;
The provision of such an electroacoustic converter in which heat losses are better dissipated in order to reduce the operating temperature of the ceramic components of the converter to increase the operating efficiency of the converter;
The provision of such an electroacoustic converter in which stress distribution and stress gradients within the ceramic components are low and in which parasitic frequencies are also low;
The provision of such an electroacoustic converter in which, because the stresses in the ceramic materials are better controlled and are lower, the converter may be of larger diameter (as compared to prior converter designs) thus allowing the use of more ceramic volume which in turn increases the power provided by the converter;
The provision of such an electroacoustic converter in which the ceramics are symmetrically arranged in the converter with respect to a vibrational node such that the front and back ceramic components generate substantially equal power and such that the motional voltages applied to the front and back ceramic components are substantially equal to reduce circulating currents and resulting losses.
The provision of such an electroacoustic converter in which, since the design is substantially symmetric and stress is more controlled, reflection of the tooling (which is typically operatively coupled to the front driver of the converter) back to the terminals is more defined and stable thus allowing a wider range of tooling stacks of various designs;
The provision of such an electroacoustic converter in which the cross sectional area of metal components (front driver, back driver, center section) in contact with the ceramics is increased thus resulting in a greater rate of heat transfer.
The provision of such an electroacoustic converter in which a metal spacer positioned between the front and back sets of ceramic components more effectively transfers heat from the ceramic components;
The provision of such an electroacoustic converter which employs a somewhat lower ceramic volume but which generates appreciably higher average power than comparable prior art transducer designs; and
The provision of such an electroacoustic converter which is of an economical design, which is of a cost efficient design, which has a long service life, and which more efficiently converts electrical energy into mechanical vibrations.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.
Briefly stated, an electroacoustic converter (transducer) of the present invention converts electrical energy into mechanical vibrations within a predetermined frequency range (e.g., ranging between about 16-100 kHz.). The converter is supplied with alternating electrical power (voltage) from a suitable power supply. The converter comprises a metal front driver mass, a metal back driver mass, a front ceramic stack, a back ceramic stack, a spacer between the front and back ceramic stacks, and a fastener coupled to the front and back driver masses for clamping the ceramic stacks and the spacer between the front and back drivers. The front and back ceramic stacks are of a suitable piezoelectric ceramic material which, when energized with alternating electrical power of predetermined frequency from the power supply that the converter is rendered resonant to vibrate in axial direction. The spacer and back driver mass are provided with fins to convect heat away from the converter to transfer the heat to ambient.